Epoxy xtrength?

Bill, if your squeezing out such cruicial amounts of epoxy that the amount that is absorbed into the paper has an effect then your dern good!

BTW, I need some vinyl cut, I'll call ya. mk

wax paper is not the best idea... you will be scraping tiny bits of wa

into the epoxy. That essentially makes voids in the epoxy bond

-- fhhuber50677

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I know. I was responding to the post of jb_brown. I'm in the market for a U Can Do .46.

Ken

Not to mention that fact that wax is partially soluble in epoxy resin, so you will be degrading the epoxy's mechanical properties. Bad idea. Avoid wax paper and waxed bathroom cups for epoxy mixing.

mjd wrote:

Strength at full cure has very little to do with cure time. Strength is mainly a function of the way it is used and the specific formulation. Epoxys can and are made to have very different cured tensile strengths for various applications. So a lot depends on which particular epoxy formulation you happen to pick up at the LHS. Hard cure is not always better. Hard cured resin also tends to be brittle. Not a big deal if you are sticking small hunks of balsa together but a real big deal if you are sticking larger hunks of hard woods together. Try glueing a chair back together with any epoxy and you will find out they are all junk for such applications. The glue joints will all usually break in a few months.

The biggest problem is usually mixing. They need to be mixed at a molecular level not the superficial mix most people give them. Mixing a small dab of one or two ccs properly is just about impossible. You wind up beating so much air into the resin that there is little chance it will be anyplace close to its potential strength. The only advantage of all these air bubbles is if the resin is a very hard and thus brittle resin the air bubbles do tend to terminate cracks. Poor mixed cured resin will have real soft regions that have nil strength and hard brittle regions also with little strength. The easiest by far to mix properly are thin resins. The typical short cure stuff is mainly so viscous that proper mixing is very difficult to impossible in small amounts. Any hand mix time of less then three minutes is going to give a poor mix. One way to estimate how long you need to mix is to watch the light refraction lines in the resin as you mix. You generally can not see this unless you are mixing at least 10 or 15 mls. You need to time how long you can see the last traces of light refraction due to the differences in refractive index of the two components. Then mix for three times the length of time it took to get rid of all visible refraction lines. The thicker the resin the longer it will take you to get adequate mix. But with thick resins you beat so much air in them so fast you can not even see the refraction lines to judge where you are in the mix process. You also can not see this refraction unless you are mixing in a clear container such as a small plastic cup.

Also, like concrete, epoxies do not cure nearly as fast as you think they do. Even once they feel hard they are still curing for a long time (days to weeks) before they reach ultimate strength. As someone else pointed out a bit of time in an oven speeds this post cure up dramatically. In fact some epoxies can only be cured in an oven as they have months or years of pot life at room temp.

All of that said, if you are not getting adequate strength out of five minute epoxy for most any appropriate model airplane application it is your own fault as you did not mix the stuff well enough to start with. The worst five minute epoxy on the market is stronger then balsa by orders of magnitude. If you get a break through the epoxy itself that is a sure sign of poor mixing. And if it broke and tore wood out that shows the wood was the weak point and stronger epoxy was not going to help a bit. If you are using it to glue large hunks of hardwood in lap joints you are simply using the wrong glue to start with. In such a case some really weak but flexable glue like gorilla glue or Titebond is far better.

Top-posting to leave your AWESOME essay on epoxies intact below.

Thanks for 'splaining it all so well.

(It's not rocket science, but there be some rocket scientists among us!)

Marty

On 17 Oct 2006 17:37:31 -0700, snipped-for-privacy@scn.org wrote in :

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I have seen a bird come apart because the former glued with a polyurethane glue came out of the joint. To me that suggested that polyurethane glues used in structural applications is marginal as the glue joints tend to be weak. However, for sheeting foam wings that stuff outshines all other adhesives I have played around with.

Yes, no and maybe all apply...

If the material being bonded is stronger than the glue... 2 hr epox will hold better than 5 min or 30 min EVERY time. Longer cure tim DOES mean the glue itself is stronger.

But in our modeling uses, the materals are typically balsa, Spruce Lite ply, Aircraft ply... all of which will break before even th cheapest (weakest) 5 min epoxy if its properly applied. All glu strength tests done to date have confirmed this.

Longer cure time gives more time to get the epoxy applied PROPERLY without mesing around with the alcohol thinning (which can prevent som epoxies from EVER curing properly) That alone makes it worth using th longer cure time stuff when building.

Longer cure time allows you to do a better, lighter job when applyin fiberglass cloth. This can make a difference in aircraft performanc when you save an ounce or two doing a .40 size model's dihedral joint.

There's no reason I can think of for using 5 min epoxy when building. For field repairs, 5 min can be very appropriate.

Don't get caught up in the speed of the glue and forget... it can tak more time to get the parts aligned correctly than what we have to wor with due to glue cure time.

Learn your glues... use the correct one for the purpose and you wil have lighter STRONGER models

-- fhhuber50677

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Thousands of lab tests have shown that pot life has little to do with the cured strength of reasonably formulated epoxys properly mixed. Much more important is what the epoxy was formulated to do. Some are purposely formulated to be weaker in tensile as this can impove the flex modulus. And in some applications that can be what you want such as in metal to metal lap joints or laminant applications with high flex stress. As I recall the off the shelf products have tensiles in the range of 7,000 to 18,000psi. No wood product has more then a small fraction of that strength perpendicular to the grain which is the usual mode of failure in a lap joint. I have personally never seen a wood joint failure with epoxy that did not tear out the wood even when used in wholly inappropriate applications like glueing a chair together.

Alcohol, even in small amounts results in very slow and poor cures. And it wrecks the tensile strength of the cured epoxy. Tensile strengths can drop to 10 or 20% of normal with major loss of flex modulus at the same time. The only possible benefit is the fully cured epoxy is so brittle it is easy to sand due to the miserable flex modulus. Much better when using epoxy in glass composites to simply use the proper epoxy and not dilute it with alcohol. Zpoxy finishing resin is very easy to use unthinned for building composites. The one exception would be when you are using the epoxy as paint. In that case strength makes no difference so it is ok to thin with alcohol if you wish. Although simply using a naturally thin epoxy to start with is easier.

I do use alcohol thinned epoxy to make permanent microscopy mounts. The reason is if it is 30% alcohol it wets out delicate micro structures fast, air bubbles float out fast without having to pull a vacuum and bond strength between the slide and cover plate is irrelevant. It also has close to an ideal index of refraction for some kinds of observations, such as 1000X mags. It is such crap strength wise it is easy to clean up what collects around or on the cover slip after it is cured. Clean up after 24 hours cure is like cleaning an almost nonadhesive rubber off the glass and if you wait a week it is so brittle it flakes right off.

the longer the set time, the stronger the bond.